1. Field of the Invention
The present invention relates to a rubber compound for a homogeneous solid golf ball and also to a solid golf ball made of the rubber compound.
2. Description of the Related Art
It has been common practice to make a one-piece golf ball or a core of a multi-layered solid golf ball (in which the core is enclosed by a cover directly or with an intermediate layer placed between the core and the cover) by heat press molding from a rubber compound composed of a base rubber such as butadiene rubber, a co-crosslinking agent such as metal salt of unsaturated carboxylic acid, a weight-adjusting agent such as zinc oxide, and a free-radical initiator such as dicumyl peroxide. The above-mentioned co-crosslinking agent (which is a metal salt of unsaturated carboxylic acid) is suitably selected from zinc methacrylate or zinc acrylate for adequate control of the hardness and rebound resilience which the resulting golf ball should possess.
The zinc (meth) acrylate is compounded in a comparatively large amount (as much as 15 to 60 pbw) into the base rubber (100 pbw) so that the resulting golf ball has desired properties. Being a fine powder, this component tends to scatter and stick to mixing rolls, thereby impeding compounding operation. In addition, it readily forms agglomerates in the rubber compound, thereby substantially reducing its ratio in the rubber compound. Moreover, agglomeration in varied degrees leads to variation in quality. In other words, the resulting golf balls vary from lot to lot in hardness and rebound resilience or vary in performance depending on the hitting position.
There have been proposed several techniques as shown below to prevent the zinc (meth) acrylate from sticking to the mixing machine and to uniformly disperse it into the rubber compound.
Patent Document 1 (Japanese Patent No. 2720541) discloses a technique for uniformly dispersing a metal salt of unsaturated carboxylic acid into a rubber compound. This technique consists of mixing together a metal salt of unsaturated carboxylic acid and a rubber in liquid form, thereby giving a master batch, and subsequently compounding the master batch into rubber compound. The disadvantage of this technique is that the resulting rebound resilience of core still has room for improvement because it employs a liquid rubber having a low molecular weight.
Patent Document 2 (Japanese Patent Laid-open No. Hei 4-314465) and Patent Document 3 (Japanese Patent Laid-open No. Hei 4-227733) disclose a technique for forming a core. This technique consists of mixing together in liquid state a metal salt of unsaturated carboxylic acid and a rubber, and subsequently forming the mixture, with solvent removed, into a core. This technique, however, needs rubber in a relatively high ratio, and hence it is not suitable for an instance where a metal salt of unsaturated carboxylic acid is required in a large amount. Moreover, this technique consists of mixing together two solutions and subsequently removing solvent, thereby dispersing a metal salt of unsaturated carboxylic acid directly into rubber. Therefore, it needs additional steps of removing a large amount solvent and drying a large amount of rubber compound, which reduces workability. This is particularly true in the case where a metal salt of unsaturated carboxylic acid is added in a large amount.
Patent Document 4 (Japanese Patent Laid-open No. Sho 59-141961) and Patent Document 5 (Japanese Patent Laid-open No. Sho 60-92781) disclose a technique of compounding rubber with a metal salt of unsaturated carboxylic acid coated with a higher fatty acid or a salt thereof. The disadvantage of this technique is that mixing temperature has to be set higher than 100° C., which is undesirable for production cost and poses a problem with scorching and loss due to scattering.
Patent Document 6 (Japanese Patent Laid-open No. 2002-85591, Patent Document 7 (Japanese Patent Laid-open No. 2002-159595), and Patent Document 8 (Japanese Patent Laid-open No. 2002-200195) disclose a technique of compounding rubber with a metal salt of unsaturated carboxylic acid coated with a thermoplastic resin. The disadvantage of this technique is that mixing temperature has to be set higher than 100° C., which is undesirable for productivity. Another disadvantage is that the thermoplastic resin used for coating sometimes reduces the rebound resilience of resulting golf balls.
Patent Document 9 (Japanese Patent Laid-open No. 2001-187167) discloses a technique of compounding rubber with an unsaturated carboxylic acid coated with a sulfur-containing compound. This technique, however, does not completely prevent the coating material from sticking to the mixing machine.
Under these circumstance, there has been a demand for a rubber compound free of lot-to-lot variation that can be produced efficiently, with a metal salt of unsaturated carboxylic acid uniformly and finely dispersed therein (even when used in a large amount) and for a golf ball with stable quality and good flight performance.